1. Field of the Invention
The present invention relates to a production method for a dielectric resonator device, such as a dielectric filter and a dielectric duplexer, in which a resonator is formed in a dielectric block.
2. Description of the Related Art
Dielectric resonator devices in which a dielectric block shaped like a substantially rectangular parallelepiped includes inner-conductor-formed holes each having an inner conductor formed on its inner surface, and an outer conductor formed on the outer surface thereof have been used hitherto as dielectric filters or dielectric duplexers.
U.S. Pat. No. 4,523,162 discloses a method for cutting the edges of an opening of each inner-conductor-formed hole with a sharp-tipped drill, which is placed in the axial direction of the inner-conductor-formed hole so that its end is in contact with the opening, in order that the periphery of the opening of the inner-conductor-formed hole serves as an open end of a resonator in such a dielectric resonator device using a dielectric block.
In the dielectric resonator device disclosed in the above U.S. patent, the inner-conductor-formed holes formed in the dielectric block are through holes of circular cross-section. However, the cross-sectional shape of the inner-conductor-formed holes is not limited to a circle. The cross section of the inner-conductor-formed holes formed in the dielectric block are sometimes substantially rectangular or substantially elliptical in order to reduce the width in the direction in which the inner-conductor-formed holes are arrayed so that multiple inner-conductor-formed holes can be arranged in a small dielectric block, or in order to increase the degree of flexibility in designing the degree of coupling between the resonators of the adjacent inner-conductor-formed holes. It is, however, difficult to apply the above production method to a dielectric resonator device that includes inner-conductor-formed holes having such a cross-sectional shape.
FIG. 10A shows openings of inner-conductor-formed holes. A cut portion 6 is formed by cutting an opening of an inner-conductor-formed hole 2a with a drill so as to separate an outer conductor 4 and an inner conductor formed on the inner surface of the inner-conductor-formed hole 2a. However, in a case in which the cutting operation is performed using a drill having a diameter larger than the cross-sectional length of the inner-conductor-formed hole 2a, when the next inner-conductor-formed hole 2b is similarly subjected to cutting, the cut portions 6 around the inner-conductor-formed holes 2a and 2b are sometimes connected. Furthermore, the cut portion 6 is sometimes substantially enlarged and reaches the next inner-conductor-formed hole 2b. In such circumstances, it is impossible to achieve a desired electrical characteristic.
In order that a cut portion will not be enlarged towards the periphery of the next inner-conductor-formed hole, a smaller-diameter drill may be moved along the edge of the opening of the inner-conductor-formed hole 2, as shown in FIG. 10B. In this method, however, a cutting machine to be used must execute control so that the drill moves in a horizontal plane while rotating. Moreover, the cutting time is prolonged, a heavy load is imposed on the drill, and the lifetime of the drill is shortened.